Adaptable servo-control system for force/position actuation

ABSTRACT

An adaptable, servo-control system for force/position actuation generally includes an electric linear actuator and a controller. The controller interfaces with the electric linear actuator and with a number of I/O signals that are independent from the electric linear actuator. The controller interface to the independent I/O signals is transparent to the manner in which the I/O signals are produced. Rather the controller simply looks for the status of the I/O that is preferably received into the controller by hard-wiring or field bus I/O messaging such as from a industrial robot or programmable logic controller. The adaptable servo-control system is particularly suited to resistance weld systems wherein the electric linear actuator can replace the pneumatic actuator providing position and force actuation of the weld tip of the welding gun.

CLAIM TO PRIORITY

This application is a continuation of U.S. patent application Ser. No.10/274,506, filed Oct. 18, 2002, entitled “ADAPTABLE SERVO-CONTROLSYSTEM FOR FORCE/POSITION ACTUATION,” the disclosure of which is herebyincorporated in its entirety by reference.

FIELD OF THE INVENTION

The present invention is related to weld motion controllers and, moreparticularly, to an electric servo-control system for welding that hascommonality from robot-to-robot and robot-to-hard-tool whether within astand alone application or, new or retrofit robotic application.

BACKGROUND OF THE INVENTION

Resistance welding is a group of welding processes in which the joiningof metal is produced by the heat obtained from resistance of the work tothe electric current, in a circuit of which the work is a part, and bythe application of pressure. The three factors involved in making aresistance weld are the amount of current that passes through the work,the pressure that the electrode tips transfer to the work, and the timethat the current flows through the work. The factor of present concernis the pressure that the electrode tips may transfer to the work. Thispressure must be precise and consistent throughout the weld cycle toassure a continuous electrical circuit through the work and prevent weldsplash. The pressure must further be consistently repeatable upon thenext iteration of the weld cycle to assure conformity among welds.

Most resistance welding performed today is done so by robots orhard-tooled fixtures utilizing pneumatic actuators to deliver thepressure required between electrode tips. The pneumatic actuation occursin a “slam open”/“slam closed” manner providing no precision control andno guaranteed repeatability of operation. Now that the advantages of anelectric servo actuator over that of a pneumatic actuator have beenrealized in the areas of precision control and repeatability, it isdesirable to provide these advantages in a process that is trulydependent upon control and repeatability, namely, welding.

However, an industrial welding environment can present multiple robottypes as well as multiple types of welding fixtures. To retrofit each ofthese robots or fixtures from pneumatics to operation with an electricservo actuator would generally require an in-depth knowledge of eachrobot's and each welding fixture's own unique components, unique controlsystem, and unique weld motion program, each requiring its own uniquemodifications thereto. The result is a complicated welding systemrunning under different programs and languages, requiring differentreplacement parts, and costing a tremendous amount of money and time.

In an ideal situation, the various robots and fixtures would each beretrofit with the same adaptable servo-control system that could makeuse of the existing pneumatic input and output signals. Each adaptableservo-control system would utilize the same components, the same controlsystems and the same weld motion programs to limit the amount of time,knowledge, and cost required. The servo-control system would presentcommonality in all applications via hardware configurations yet beadaptable, through software configurations, e.g., adjustment of programparameters and recognition of existing pneumatic input and outputsignals, to accommodate the differences in applications.

SUMMARY OF THE INVENTION

The needs described above are in large part met by the adaptable,servo-control system for force/position actuation of the presentinvention. The servo-control system generally includes an electriclinear actuator and a controller. The controller interfaces with theelectric linear actuator and with a number of I/O signals that areindependent from the electric linear actuator. The controller interfaceto the independent I/O signals is transparent to the manner in which theI/O signals are produced. Rather the controller simply looks for thestatus of the I/O that is preferably received into the controller byhard-wiring or field bus, e.g., DeviceNet I/O messaging or Profibus,such as from a industrial robot or programmable logic controller. Thecontroller utilizes the I/O signals to select from a number ofpre-established parameters to generate a motion profile for closedloop-controlled, position actuation and/or force actuation of theelectric linear actuator.

In an alternative embodiment, the controller interfaces with andcontrols more than one electric linear actuator but does so through useof a single motion control program. The single motion control programincludes a number of characterizable parameters, each of which can beindependently characterized for each electric linear actuator thecontroller is to control. The characterizable parameters are preferablyuser-entered/selected through a single programming device that isconnectable to the controller. In a preferred embodiment, this singleprogramming device is a portable, programming pendant. In utilizing theI/O signals and character parameters, the controller preferably createsa bit-mapped table that is referenced by the motion control program,i.e., the bit-mapped table maps input signals to specific position orforce actuations of the linear actuator.

The servo-control system is particularly suitable to resistance weldingsystems, whether fixture systems or industrial robot systems, where theelectric linear actuator can be used to position or force actuate thewelding tip of the welding gun. Again, the controller can interface withmore than one electric linear actuator to effect different andindependent weld operations per each electric linear actuator and canuse a single motion control program with characterizable parameters pereach linear actuator. The servo-control system may additionally monitorweld tip wear through various programmed operations and can observe thesafety, stop operation features of a robotic welder by linking into thatfeature, i.e., using the controller to monitor the I/O signal enablingoperation of the robot.

The controller of the servo-control system generally includes a computerand a motion controller/drive that is operably coupled to the computer.The computer receives the characterized parameters and downloads theparameters to the motion controller/drive, which then performs theoperation of closed loop control of the electric linear actuatoraccording to the hard coded program. The controller may include morethan one motion controller/drive, and preferably includes one motioncontroller/drive per each linear actuator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the adaptable, servo-control system forforce/position actuation system (weld axis system) of the presentinvention.

FIG. 2 depicts an assembly line situation making optimal use of theadaptable, servo-control system for force/position actuation system ofthe present invention by interfacing with six-axis robots and asindependent weld gun fixtures.

FIG. 3 is a block diagram of a weld axis controller.

FIG. 4 depicts a high-thrust electric servo linear actuator.

FIG. 5 depicts the actuator of FIG. 4 utilized within a scissor weldgun.

FIG. 6 depicts the actuator of FIG. 4 utilized within a C-clamp weldgun.

FIG. 7 is an example configuration of a programming pendant.

FIG. 8 is an example of a bit-mapped table that may be referenced by themotion control program.

FIG. 9 is a flow chart depicting the preferred sequence of operations ofthe weld axis system.

FIG. 10 is a flow chart depicting the operations performed by the weldaxis system during the actual weld process.

FIG. 11 is a flow chart depicting the operations performed by the weldaxis system during non-weld operations.

FIG. 12 depicts the preferred weld motion profile of the weld axissystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

I. System Overview

The present invention comprises, an adaptable servo-control system forforce/position actuation in various welding applications includingresistance welding, and more specifically, spot welding, applications.In other words, the present invention operates as an independent “weldaxis” system. The adaptability of the weld axis system enables thepresent invention to operate as an interface to existing six-axis robotsin both new and retrofit situations, as well as a stand-alone, weldinggun fixture. In all situations, the weld axis system is substantiallyambivalent to the control scheme and/or programming language in anexisting robot but is able to use those inputs/outputs typically used bya pneumatic valve for weld gun actuation to control an electric linearactuator to deliver the desired motion profile and squeeze force thatwould typically have been delivered by a pneumatic actuator.

Referring to FIG. 1, a schematic diagram of the weld axis system 20 ofthe present invention is provided. As indicated, the weld axis system 20generally comprises the elements of a control system, a.k.a., a weldaxis controller 22, an electric servo linear actuator 24, and aprogramming pendant 26. The motion of the electric servo linear actuator24, under direction of the weld axis controller 22, provides thepositioning and squeeze force required of the weld gun 28 (see FIG. 2);the current delivered by the weld gun 28 to effect the weld is undercontrol of a separate weld controller (not shown) that is triggered fordelivery by the robot or fixture. The programming pendant 26 enables auser of system 20 to setup and characterize the operation of the weldaxis system 20 for a specific application.

FIG. 2 presents just one of many possible scenarios of the potentialcombinations and uses of the weld axis system 20. Specifically, FIG. 2depicts an assembly line, multi-weld situation that is controlled by amaster PLC (programmable logic controller) 29 wherein to the left sideof the figure, three six-axis robots 30 have each been individuallyinterfaced via their controllers 32 to a weld axis controller 22. TheI/O indicating requests for weld motion from the robot, e.g., those thatwould typically be provided to a pneumatic actuator, are provided fromthe controller 32 to the weld axis controller 22. The transfer of I/Obetween the controller 32 and the weld axis controller 22 is preferablythrough some sort of field bus such as Profibus or DeviceNet I/Omessaging.

In the left-side configuration of FIG. 2, only one electric servo linearactuator is controlled per weld axis controller 22. However, referringto the right-side configuration of FIG. 2, the weld axis controller 22has been configured to cluster and control from a single weld axiscontroller 22 a plurality of electric servo linear actuators 24 withinweld guns 28. Each electric servo linear actuator 24, whether undercluster control or single control, is able to be independentlyprogrammed for desired weld motion through use of programming pendant26. The single programming pendant 26 is communicatively connectable toeach weld axis controller 22, preferably through an RS-232 connection34, for individual setup and characterization for the operation of eachelectric servo linear actuator 24.

Further, with regard to the right-side configuration of FIG. 2, it canbe seen that the weld axis system 20 may be implemented in conjunctionwith a six-axis robot 30, new or existing, and may also be implementedas a stand alone, welding gun fixture 36. In the instance of a weldinggun fixture 36, the I/O indicating requests for weld motion are receiveddirectly into the weld axis controller 22 either from the master PLC 29,via field bus I/O messaging, or through hard-wired I/O.

II. System Components

Referring once again to FIG. 1, and to reiterate that from above, theweld axis system 20 generally comprises the elements of a controlsystem, a.k.a., a weld axis controller 22, an electric servo linearactuator 24, and a programming pendant 26. The motion of the electricservo linear actuator 24, under direction of the weld axis controller22, provides the positioning and squeeze force required of the weld gun28 (see FIG. 2); the current delivered by the weld gun 28 to effect theweld is under control of the robot or fixture. The programming pendant26 enables a user of system 20 to setup and characterize the operationof the weld axis system 20 for a specific application.

II.A. System Components—Weld Axis Controller

FIG. 3 provides a block diagram of the weld axis controller 22. The weldaxis controller 22 generally includes a transformer 40, a computer 42,and at least one motion controller/drive/PLC (MCD) 44. FIG. 3 depicts anembodiment of the weld axis controller 22 wherein three MCDs 44 areutilized. Such a configuration would be suitable to the right-sideconfiguration of FIG. 2 wherein the control of three electric servolinear actuators 24 has been clustered. It should be noted, however,that any number of MCDs 44 may be used within the weld axis controller22, if suitable to the application, without departing from the spirit orscope of the invention.

The transformer 40 is well known in the art and operates to convert theintake AC voltage to a level suitable for powering the MCDs (which inturn power the electric servo linear actuators 24). In an alternativeembodiment, the transformer need not simultaneously power all MCDs 44but may be set up to power a single MCD on a timed basis enabling theuse of a smaller transformer.

The computer 42 is preferably a low-cost computer whose main function isto act as a conduit between the programming pendant 26 and the MCDs 44.The computer 42 requests information via the programming pendant 26through various prompts and tables presented to the user at the pendant28. Beyond receiving data entered through the programming pendant 26,the computer 42 is further capable of processing calibration data, ofusing the data to establish register values, and of downloading theseregister values to the appropriate MCD 44 as well asuploading/downloading changes in I/O, preferably through the use ofASCII serial port commands or other appropriate means. The computer 42may be provided with an optional ethernet connection, other wired orwireless connection, to enable connection of the computer 42 to a remotecomputer or network. Note, that in an alternative embodiment, thefunctions performed by the computer may be implemented partially orentirely within the MCD 44 by making the appropriate hardware andsoftware modifications to the MCD 44.

The MCD 44 preferably performs three functions: 1) a motion controller;2) a drive; and 3) a programmable logic controller (PLC). In itsperformance as a motion controller, the MCD 44 preferably providescommands for absolute, incremental, and velocity moves, time delays,wait-on conditions/inputs, output/flag control, parameter value changesincluding torque limit, following error, position band, trigger pointsfor in-motion changes and maximum velocity. Of course, other commandsmay be provided without departing from the spirit or scope of theinvention. Further, in its performance as a motion controller, the MCD44 preferably provides for event triggering based on intermediatepositions as well as motion pause and resume.

In its performance as a drive, the MCD 44 preferably utilizes spacevector commutation for efficient bus voltage utilization and optimalspeed/torque curves. The MCD 44 also preferably utilizes flux vectorcurrent control to enable accurate high bandwidth control of torqueproducing current for high efficiency and maximum torque over the fullspeed range of the drive. The MCD 44 also preferably makes accommodationfor a drive enable input.

In its performance as a PLC, the MCD 44 preferably provides for areal-time scan supervisory function that is continuous from power-upalong with a typical scan-time of less than 2 milliseconds. The MCD 44also preferably provides the option of ladder logic programming, e.g.,providing 175 rungs of logic, up to four lines deep with five inputoperations and one output coil. The operations of the ladder logicpreferably include normally-open, normally-closed, logical invert,one-shot timer, output coil, latch, unlatch, timers and counters, aswell as register transfers and compares. Internal bit-flags forinformation transfer between the motion controller and the PLCfunctions.

A product generally meeting all of the criteria outlined above for theMCD is the Tol-O-Matic AXIOM® PLUS PV SERIES BRUSHLESSSERVO/CONTROLLER/DRIVE/PLC, available from Tol-O-Matic, Inc., of Hamel,Minnesota. The User Manual for the AXIOM® PLUS, identified bypublication number 3600-4628_(—)01, is hereby incorporated by referencein its entirety. At the current time, features specific to the presentapplication are provided by firmware and software identified throughVersion number 1.04d(IP).

In the most basic of terms, the MCD 44 powers and provides servo controlof the electric linear actuator 24. Specifically, the MCD 44 providesthe actuator 24 with power and receives from the actuator 24 a positionfeedback signal to achieve closed loop control of the actuator 24according to a desired weld motion. The commutated power signal isgenerated by a program stored in the memory of the MCD 44 and may beinitiated by external I/O or internal operating parameters. The programwithin the memory of the MCD has been placed by first collecting andprocessing information within the computer 42 then downloading theinformation to the memory of the MCD 44.

II.B. System Components—Electric Servo Linear Actuator

The electric servo linear actuator 24 is preferably a rod screw actuatorand is selected dependent on the application and the weld forcerequired. The preferred rod screw actuator is the Tol-O-Matic HT Seriesactuator, an assembly drawing is provided in FIG. 4. As shown, the HTseries rod screw actuator generally includes a thrust rod 50 having amachined rod end 52. The thrust rod 50 is coupled to a lead screw 54 viaa bearing assembly 56 and nut assembly 57. A shaft 60 of the lead screw54 passes through a bearing plate 61 and second bearing assembly 62. Theshaft 60 continues through a spacer 64 and third bearing assembly 66 tosupport the windings 68 of an actuator-integrated motor. A housing 70 isprovided about the windings 68 and enables an encoder 72 to be coupledto the shaft 60. A cylinder body 74 houses the components of theactuator 24 while an end cap 76, providing wiring access for power,control signal, and encoder feedback, creates a sealed unit. At thecurrent time, the Tol-O-Matic HT series of actuators are comprised ofthree actuators including the HT7, HT12, and HT23 wherein each iscapable of providing up to 700, 1200, and 2300 lbs. of weld force,respectively. The Tol-O-Matic HT series actuator is described in detailin U.S. Pat. No. 6,756,707, which is hereby incorporated by reference inits entirety.

The integrated motor design of the HT series actuator eliminates theneed for a coupler and provides for overall decreased weight andfootprint. Specifically, the compact package of the actuator 24 enablesit to fit most pneumatic cylinder footprints making it particularlysuitable to retrofit robotic applications as well as virtually all otherresistance welding robotic or fixture applications. FIG. 5 depicts atypical pneumatic, scissor weld gun 80 wherein the location normallyreserved for a pneumatic actuator has been easily replaced by theelectric servo linear actuator (HT series actuator) 24 enabling theelectrode tip 82 to reliably and repeatedly deliver the desired weldforce. FIG. 6 demonstrates how the electric servo linear actuator 24 canjust as easily replace the pneumatic actuator in a pinch or C-stylewelding gun 84, once again enabling the electrode tip 86 to reliably andrepeatedly deliver the desired weld force.

It should be noted that while the Tol-O-Matic HT series actuator is thepreferred electric servo linear actuator 24 of the present invention,other types of electric servo linear actuators may be used withoutdeparting from the spirit or scope of the invention.

II.C. System Components—Programming Pendant

The programming pendant 26 is a portable pendant that may be used forthe set-up and/or re-characterization of any weld axis controller 22. Anexample of a possible configuration of the programming pendant isprovided in FIG. 7; of course, other programming pendant configurationsmay be used without departing from the spirit or scope of the invention.The programming pendant of FIG. 7 is provided with an RS-232communication connection 88 enabling quick access and communication withthe weld axis controller 22. The programming pendant 26 is also providedwith a visual interface 90 and a tactile interface 92. The visualinterface 90 provides for visual prompting of the user while the tactileinterface provides a means for user data entry or selection.

III. System Operation

The setup of all weld axis systems 20, whether a robotic or hard-toolfixture application, is preferably performed through use of a single,pre-established weld motion program. The single weld motion program isadjusted for specific weld applications only by values stored inregisters during the setup and characterization of the weld axiscontroller 22. These values are initially obtained by the computer 42 ofthe weld axis controller 22 through user interaction via the programmingpendant 26. Upon completion of the setup and characterization, the nowdefined, specific weld application registers are downloaded to the MCD44 of the weld axis controller 22 whereby the MCD 44 provides continuousclosed servo loop control of the electric linear actuator 24.

III.A. System Operation—Setup and Characterization

The operation of a specific MCD 44 and its connected linear actuator 24is achieved through its programmed setup and characterizationparameters. These parameters are preferably entered through theprogramming pendant 26 by prompting the user with a plurality ofuser-interactive, menu-driven screens that include the following menuselections: 1) Setup; 2) Current vs. Force; 3) Define Close; 4) OpenPosition; and 5) Weld Schedule.

III.A.i. System Operation—Setup and Characterization—Setup Parameters

Selection of the setup menu option preferably prompts the user to entera series of parameters that will help define the overall operation ofthe weld axis system 20. Specifically, the user is prompted to enter themodel of the actuator 24. In the preferred embodiment of the presentinvention, the single weld motion program has been pre-programmed torecognize a plurality of linear actuators, e.g., a plurality ofTol-O-Matic electric servo, linear actuators such as the HT Seriesactuators, and to recall the fixed operating parameters/characteristicsassociated therewith. This feature eliminates the need for the user toenter the non-adjustable, non-changing parameters of the actuator. Thevarious operating parameters of the actuators that are pre-programmedand recallable preferably includes the number of encoder counts per inchof travel of the actuator, the maximum torque the actuator can provide,etc. Of course, a user may also simply enter the various operatingparameters should recall of an existing actuator be possible ordesirable.

Further, within the setup menu selection, the user is also prompted toenter the measurement units that will be used, e.g., imperial or metric,as well as to define the allowable tip wear, e.g., {fraction (1/4)}inch, and the thrust capacity, i.e., the maximum thrust the gun candeliver (in the instance of a rod screw linear actuator, such as theTol-O-Matic HT Series actuators, the thrust capacity is the axial loadwhich the screw and nut can deliver to the rod). The user is furtherprompted to select the tip closed direction and to enter the gunreduction. The gun reduction is generally defined as a ratio of the armlengths of the welding gun, wherein the first arm length of the ratio isthe weld tip to pivot point distance and the second arm length of theratio is the actuator point of contact to pivot point distance. By wayof example, a gun reduction of two would mean that for a 1,200 lb.thrust at the weld tips, the actuator would have to supply 2,400 lbs. offorce.

Finally, within the setup menu selection the user is prompted toestablish the actuator position limits (tips not present). By selectingthe actuator position limits option, the MCD 44 initiates movement ofactuator 24 in the close direction until a hard stop is reached. Thisposition value is recorded in a register as a position limit that shouldnever be exceeded thereby protecting the actuator.

III.A.ii. System Operation—Setup and Characterization—Current vs. ForceParameters

Upon selecting the current vs. force option from the menu via theprogramming pendant 26, the user is provided with the opportunity tocalibrate the torque output to the force seen by the weld gun. In thepreferred embodiment, the weld motion program utilizes ten percentincrements of torque output. At each ten percent increment, the MCD 44sets the respective torque while the programming pendant 26 prompts theuser to enter in, using a force gauge for measurement, the measuredvalue that the given torque produces. The weld motion program continuesthe ten percent increment until the previously entered thrust capacityof the gun is reached or 100 percent torque is delivered. From thiscorrelation, any force that is later entered into the weld motionprogram will automatically be converted by the program to a desiredcurrent in the drive.

III.A.iii. System Operation—Setup and Characterization—Define CloseParameters

Upon selecting the define close option from the menu via the programmingpendant 26, the MCD 44 initiates motion to close the welding tips. Afterthe close motion has been completed, the programming pendant 26 operatesto prompt the user to enter whether the welding tip is a new tip. If itis indeed a new tip, the weld motion program accesses the previouslyentered allowable tip wear value as a reference point for the weldsenabling monitoring of tip wear. As such, at a desired time, the usermay connect the programming pendant 26 to the weld axis controller 22 toaccess a desired MCD 44 and trigger an input to request a check of tipwear. A close is then initiated by the MCD 44 and a comparison is madeby the MCD 44 between the current closed position and the initial “newtip” closed position. Alternatively, the MCD 44 may be programmed toreceive an input request from a non-pendant, external device, e.g.,master controller, for a tip wear check. The MCD 44 can be programmed toperform the check in response to the input and to provide an output tothe external device indicating whether or not the allowable tip wear hasbeen exceeded.

The result of the comparisons, i.e., the difference between the twopositions, can then be utilized by the MCD 44 for two purposes. Thefirst purpose is to tell the user if the allowable tip wear limit hasbeen reached and the second purpose is to adjust the weld motion profileof the weld motion program within the MCD 44. Upon detecting wear of thetip, the initial point of deceleration of the actuator is extended; themore wear, the more the deceleration is extended. Extending the initialdeceleration point keeps the low speed approach time to a minimumthereby maximizing weld cycle time.

III.A.iv. System Operation—Setup and Characterization—Open PositionsParameters

Upon selecting the open positions option from the menu via theprogramming pendant 26, the user may define a plurality of positions asopen gun positions based on a binary output received from a hard-wiredoutput, robot controller, or master PLC. These open gun positions aregenerally used by the weld motion program to move the weld tip to anopen position after a weld has been completed and/or as a defaultposition when no motion is occurring, and are preferably maintained in asimple look-up table format. The received binary output is used by theweld motion program to point to a certain value within the table causingthe MCD 44 to initiate a corresponding open motion in the actuator.

III.A.v. System Operation—Setup and Characterization—Weld ScheduleParameters

Upon selecting the weld schedule option from the menu via theprogramming pendant 26, the user may define a plurality of forces andcorresponding material thickness based on a binary output received froma hard-wired output, robot controller, or master PLC. From thecalibration that occurred in the “current vs. force” option, the forceselected by the received output can then be converted to a currentsetting for the actuator drive. The material thickness selected by thereceived output is used by the weld motion program within the MCD 44 tocalculate a deceleration point to slow down from a fast traverse to afinal, contact velocity. This calculation is based upon the previouslydefined close position.

As such, deceleration does not always start at the same point withoutregard to the tips and/or the material thickness. Rather, the maximummaterial thickness (previously entered and stored) is subtracted fromthe closed position, as is the deceleration distance, to produce thepoint at which deceleration is to start. By utilizing this calculation,the wasted time that would result from slow contact speed if thedeceleration took place too early is eliminated. The tip wearinformation is preferably similarly used to add distance to begindeceleration as the tips wear.

III.B. System Operation—Bit Mapping

As referenced in the setup and characterization description in theparagraphs immediately above, the weld motion program utilized by theMCD 44 preferably takes advantage of bit-mapped tables for itsoperation. An example of a bit-mapped table is provided in FIG. 8. Inthis example, the weld motion controller utilizes four weld inputs (fourthat are typically associated or had been previously used with pneumaticactuation), the combinations of which correspond to a specific force andmaterial thickness, which corresponds to a weld motion programoperation. Specifically, when weld input four is high, a tip seatoperation has been requested of the weld motion program. The tip seatoperation has a 1,200 lb. force and 0 inch material thickness associatedwith it. These force and material parameters are accessed by the weldmotion program so that the appropriate control signal may be sent fromthe MCD 44 to the actuator 24 to achieve the desired operation. Tipdress and force check operations may be similarly requested.

A position/location operation may also be requested of the weld motionprogram by receiving a binary output from a robot (or PLC or hard-wiredoutput). In the instance of receiving binary robot output 341, the weldmotion program knows that it is associated with the operation of movingto a location A, which corresponds to bit two of the weld input bitsbeing high and bits one, three, and four being low. When bit two is highand the other bits low, the weld motion program knows that a force of1,000 lbs. and a material thickness of 0.050 are the parameters todefine the force and motion of the actuator, and a control signal isgenerated by the weld motion program to achieve the desired action.Locations B, C, and D may be similarly requested.

Through the bit-mapping described above, a user is able to use the I/Oto select parameters that will optimize the motion profile of theactuator without having to enter the inner programming language of thecontroller. The user is not required to enter a plurality of differentprofiles for the actuator based on material thickness, closed positionand open position, rather the profiles are automatically generatedthrough the easily generated, user-defined bitmap (as entered throughthe programming pendant).

Continuing with the example, and the table of FIG. 8, the weld motionprogram utilizes two open inputs, an enable input and a close input. Thevarious combinations of the open input bits corresponds to a requestedposition upon which the weld motion program may act. The enable inputand close input are preferably used for system safety. Specifically, itshould be noted that in the instance of the weld axis system being usedwith a robot, all safety issues remain in control of the robot with theweld axis system linking into those safety precautions provided by therobot. In particular, it is an output from the robot (appearing as theclose input) that initializes motion within the weld axis system,without it the weld axis system does not move.

Further, with regard to safety within a typical welding work cell, anemergency stop (estop) safety circuit is included. When that estop istriggered, the circuit preferably trips contactors in the MCD that firststops the motion of the actuator and then disconnects the power to themotor of the MCD. The drive/control portion of the MCD remains energizedto maintain the encoder position of the actuator. The estop safetycircuit is preferably hardwired to a time delay contactor. The contactorhas one output that opens immediately; this output is used to stop themotion through an input to the MCD. The contactor has a second outputthat has a selectable time delay that gives the MCD time to stop motionbefore it opens the motor power connections.

A typical welding work cell additionally includes a live man switch,i.e., when a user enters the work cell, he must hold the live man switchin order to produce any motion. When used with the present system, thelive man switch must be depressed for motion of the actuator and whenthe live man switch is dropped out the MCD, as with the estop, firststops the motion of the actuator and then disconnects the power to themotor of the MCD. Meanwhile, the drive/control portion of the MCDremains energized to maintain the encoder position of the actuator.

It should be noted that while the above bit-mapping is described withreference to digital I/O, in an alternative embodiment analog I/O may beused.

IV. System Operation—Program Sequence

The preferred sequence of operation of the weld axis system of thepresent invention is diagrammed in the flow charts of FIGS. 9, 10, and11. Of course, other and/or additional operational sequences may beimplemented with the weld axis system without departing from the spiritor scope of the invention.

Per FIG. 9, the weld axis system operation 100 begins, per decisionblock 102, by determining if the actuator is in its defined homeposition. If the actuator is not at its home position, a homing routineis preferably performed, per operations block 104, whereby the actuatoris returned to its home position. Once at a home position, the programwithin MCD determines, per decision block 106, whether it has receivedan instruction that a weld is to be performed. If a weld is to beperformed, the sequence of operation for the weld process begins peroperations block 108, further detailed with reference to FIG. 10.

Per FIG. 10, the weld process 108 of the weld axis system begins by theactuator moving the tip to a position 0.125 inches from maximum materialthickness (previously entered parameter) to weld, per operations block110. The actuator then presses the tip against the material at apredefined torque (previously entered parameter), per operations block112. The program within the MCD then questions whether the position ofthe tip, for this weld force, has changed from the last time executed bymore than a maximum single weld tip burn off (a parameter that may beprogrammed by the user or, more preferably, hard coded), per decisionblock 114. If yes, a signal is readied to be sent from the MCD throughthe computer to the weld controller that a weld should begin, per datablock 116. However, prior to delivering the signal, the program withinthe MCD determines if a desired torque, as delivered by the tips, hasbeen reached, per decision block 118. If the desired torque has beenreached, the signal to begin the weld is delivered to the weldcontroller, and the MCD awaits the signal from the weld controller thatthe weld has been completed, per block 120. Upon completion, control isreturned to the weld cycle 100 of FIG. 9. It should be noted that themanner in which the actuator moves during the weld process, e.g., fromopen to close position and close to open after delivery of the desiredtorque, is preferably defined by a motion profile. The preferred motionfor profile for the actuator of the weld axis system is described indetail below in Section V.

If the answer to decision block 114 or to decision block 118 is no, asignal is sent from the MCD through the computer to the weld controllerthat an error has occurred and no weld should be delivered, per datablock 122. Upon delivering the signal, control is returned to theoperational sequence of FIG. 9.

Referring once again to FIG. 9, if the answer to decision block 106 isno or if control has been returned from the weld process 108, theprogram within the MCD questions whether an instruction has beenreceived to move to an open position, per decision block 124. If theinstruction has been received, the actuator moves the tip to an openposition, per operations block 126. Once the open position has beenachieved by the actuator, or if the open instruction was not received,the program within the MCD questions whether there is a request toperform a special function, per decision block 128. If there is norequest for performance of a special function, the program within theMCD returns to decision block 106 thereby questioning once again whethera weld is to be performed.

If there is a request for performance of a special function, the specialfunction operations begin, per operations block 130 and control of theprogram within the MCD proceeds per the flowchart of FIG. 11. Generally,the special functions portion of the program are those operationsdescribed in detail in section III.A. above, i.e., SystemOperation—Setup and Characterization. However, note that sectionIII.A.iv. regarding open position parameters and section III.A.v.regarding weld schedule parameters have been omitted from FIG. 11 forclarity since they involve no movement of the actuator but only userentries and computer calculations.

As such, the special functions depicted in FIG. 11 include, per decisionblock 132: 1. define limit—the sequence by which actuator positionlimits and weld gun position limits are established (described inSection III.A.i above); 2. Calibration—calibrating the torque output tothe force seen by the weld gun (described in Section III.A.ii. above);3. Define Close—the point at which the weld tips reach a hard stop(described in Section III.A.iii. above); and 4. Tip WearCheck—determining the wear that has occurred to the tips based on thedefined close position (described in Section III.A.iii above).

In the instance of the selection of choice number 1 (“define limit”),the actuator moves itself or the tips to a hard stop, per operationsblock 134, wherein the position of the actuator is recorded within theprogram as a never exceed limit, per operations block 136. Selection ofchoice number 2 (“calibration”) results in the actuator moving to aposition to gauge thickness, per operations block 138. Torque is thenapplied through the actuator at predefined increments until apre-established limit is reached, per operations block 140. The actuatoris then moved to an open position, per block 142.

Selection of choice number 3 (“define close”) results in the actuatormoving the weld tip until a hard stop is reached, per operations block144. Selection of choice number 4 (“tip wear check”) results in theactuator moving the weld tip to a closed position, per operations block146, wherein the program determines whether the position of the tip isbeyond a pre-defined wear position, per decision block 148. If the tipis beyond a pre-defined wear position, a signal is sent that the max tipwear has been reached, per operations block 150.

Upon completion of all of the special functions, the operation of theweld axis system returns to the operations sequence of FIG. 9.

V. System Operation—Motion Profile

The preferred motion profile, i.e., the definition of an objectsposition and velocity relationships in time during a move, used by theweld axis system 20 for movement of the weld tip during a weld cycle isshown in FIG. 12. A table describing each segment of the motion profilewith typical values, e.g., accelerations/decelerations, initial andfinal velocities, time, and distance traveled is provided. Of course,the values are provided by way of example only and are dependent uponthe application of the weld axis system and the program of the weld axissystem as set-up and characterized by the user.

As indicated, segment one of the motion profile indicates the actuatoraccelerating the tip to a desired velocity to initiate closure of thetips. Per segment two, the desired velocity has been reached and ismaintained until a deceleration is desired to effect tip closure. Persegment three, the actuator decelerates to a desired tip closingvelocity. Per segment four, the desired tip closing velocity has beenreached and tip contact is made, per segment five. During segment six,the welding gun arms deflect and the actuator decelerates to a stop. Persegment seven, welding occurs while the actuator is held in position.Per segment eight, the actuator receives the instruction to open thetips and accelerates at high speed to a desired tip opening velocity.Per segment nine, the desired tip opening velocity has been reached andis maintained until a deceleration is desired to complete the tipopening. Per segment ten, deceleration is begun to complete the tipopening process. And, finally, per segment eleven, the tip opening hasbeen completed by the actuator and the actuator waits in position for anew command to close the tips.

The present invention has been described with respect to particularillustrative embodiments. It is to be understood that the invention isnot limited to the above-described embodiments and modificationsthereto, and that various changes and modifications may be made by thoseof ordinary skill in the art without departing from the spirit and scopeof the appended claims.

1. An adaptable servo-control system for position and/or forceactuation, comprising: an electric linear actuator; and a controller,wherein said controller interfaces with said electric linear actuatorand with a plurality of I/O signals that are independent from saidelectric linear actuator, wherein the interface between said controllerand said plurality of I/O signals is transparent to the manner in whichsaid plurality of I/O signals are produced, wherein said controllerutilizes said plurality of I/O signals to select a plurality ofparameters to generate a motion profile for closed loop-controlled,position and/or force actuation of said electric linear actuator.